10.5:1 single vs 11.5:1 performance
 

Anyone care to comment on streetable performance results on engines at 10.5:1 single plug 3 litre engines (think SC RS 250-275 hp) vs 11.5:1 twin plug 3 litre engines. Assume same 83 ROW head ports, SSIs and M&K 2-2 mufflers. Assume Weber intakes on both for discussion purposes, though will be EFI in actual execution.

3 litre in both cases, same exhaust in both. Car is lightweight 1971 'daily driver' targa that sees 4000 kms per year in the dry only.

So I know the answer is generally that twinplugging doesnt get youestra power, only more safety margin. But given the same margin what's the difference other than ~$700?

This doesn't directly answer your question, but this may be a good spot for some general bits of info.

nugget a)

Twin plugs allow combustion to start at two separate points within the combustion chamber. This allows flame propagation to be much quicker, resulting in a faster, more complete, and more efficient burn.

nugget b)

As compression increases, so does the air-gap resistance across the spark plug electrode. For small increases in compression, you may not notice significant change, however with higher compression increases it will be necessary to increase the output of the ignition system to compensate for the increased resistance. Failure to compensate will result in reduced spark plug efficiency.

One point of compression is generally worth around 4% hp.

Thanks Cash. I've done a twinplugger before (say that out loud with a straight face!) with good results and appreciate its necessity at high compression. I think the consensus seems to be over 10.5:1 its necessary.

Paul, do you mean 4% per decimal point (as in the '5' in 10.5:1) or in the integer (the '10' in 10.5:1)?


john

What kind of fuel are you going to use? Unleaded race gas?

I hope you have better gas than our burnable water that they sell here at the pump.

I belive this is one of the biggest issues to compression and streetability, at least in the PacNorthwest.

Randy Jones
1971 911 "Iris"
298 HP / short stroke 3.2 / twin plug / 10.27 : 1 compression
(running on pump gas)

One point of compression is 1.0 (i.e. going from 8:1 to 9:1).

I run pump gas on a 2-plugged 3.2L carb'd motor and have less than 11:1 comp. ratio - it pays to be conservative. Gas in a back water - along a nice fun back road - may be even worse than in a bigger market...

Benefits:
can give up to 7% extra hp (esp. on earlier motors)
can run higher compression & retarded spark for better combustion, more power (i.e less advanced spark)


the bigger the chamber diameter, the more imp. it is to use 2 plugs...

Thermodynamics says that the engine efficiency (and therefore the power) goes as 1-R^(-2/5), where R is the compression ratio. In going from 10.5:1 to 11.5:1, the power should ideally go up by a factor of 1.023, an increase of 2.3%. You don't win quickly by increasing the CR, especially at higher CR. The curve starts out steep, but levels off fast. For example, the difference between 8:1 and 9:1 is a factor of 1.036 a 3.6% increase in HP. I have a 3 page write up on the subject, if anyone is interested.

I'm interested!

iwp4 - at - comcast - dot - net

So, the Nat'l Labs are now working on higher CR's rather than nukes, or Lithium/solar power plants?

Thanks Guys. Yes Scott, I'd like to see. Can/would you post it?

Increasing the compression ratio allows the engine to reach rpms quicker, however it is the least dramatic thing you can do to increase h.p in an engine. IMO of course.

The national labs are working on energy in general, although I'm not. I did this in my spare time just for fun. I can't figure out how to post a pdf. If anyone can tell me, I'll do it. Otherwise, I'll email it to people who ask. I'm travelling, so it will be a few days.

I've sent it to souk and randywebb. SEnd me a PM with your email if you want a copy.

Thanks.

The ideal compression ratio for a porsche is 10.3 to 1. The closer your are the better. All of porsche race cars through the years ratios have been 10.3 to 1.

J

I am not sure when Porsche raised the C/R to 11.3:1 but my '97 3.6 has that C/R and the 996 and 997 motors are 11.3:1.
The original 3.2 [early '89] engine was 9.5:1. When it came time to rebuild, twin plugging the engine and going to 3.4 was out of the question because the smog laws on the 'left coast' are bad and getting worse. Going to a 3.4 wasn't the issue. The smog issue is that when the state starts to turn the screws on older cars [especially air cooled ones] a twin plugged engine will fail the 'visual' test. It won't even make it to the 'sniff test', they will just fail it because everything [including the filter box] from the intake to the outlet of the cats must be stock, original equipment.
Anything larger than a 3.2 [or a higher C/R] must be twin plugged because the length of travel of the spark across the cylinder is too great. This is why all of the later [964 and up] engines are twin plugged.
Drums

i'm in the low 10:1 c/r with 100mm P&C's running twin plugs...took a surprising amount of total advance to get peak output.... gonna hit the dyno again with new headers and to double check the readings.

I do not think there is any single "ideal" compression ratio.

The optimum CR will vary based on race rules, state & federal regulations, and the particular fuel use, along with combustion chamber design, boost (if any), cam, and etc.

I haven't read the paper above yet, but I expect it to shed some light on this issue.

"Anything larger than a 3.2 [or a higher C/R] must be twin plugged because the length of travel of the spark across the cylinder is too great."

OR!!!

1. Use super high octane gas, i.e. race gas, AND compromise the max.
timing possible, i.e. 40-45 degrees - 964/993, or
2. Super retard the max. timing.

Both 1 & 2 above result in less than optimum torque.

"travel of the spark across the cylinder"

Actually, it's the length of propagation of the flame front causing excessive
pressure buildup resulting in detonation, i.e for the single side positiioned
spark plug versus a centrally located spark plug - 996/997 etc.

Bottom line: Even a stock 911 CR will benefit from twin plugging because
it will allow a greater advance curve without the higher potential for detonation
of the single sided 911 spark plug location.